1. Field Of The Invention
The present invention relates generally to machine tools and, more particularly, to a device for tightening or loosening the draw bar of the machine tool.
2. Description Of The Prior Art
Milling machines are well known in the prior art. One such machine is described in Manual No. M-105, published by Bridgeport Machines, Bridgeport, Conn. As is typical in such machines, the cutting tool is held in a collet (or tool holder), which in turn is held in a spindle which is rotated directly by the machine. The collet is held in place by a draw bar, one end of which threadably engages the collet to hold it tightly against the spindle.
In order to change cutting tools, it is necessary to rotate the draw bar, while locking the spindle against rotation. Such action loosens the collet and releases the tool. Further rotation of the draw bar allows the collet to be removed and replaced. To replace the tool and/or the collet, the draw bar must be rotated in the opposite direction to "draw up" the collet to its proper location and to tighten the collet about the tool.
The collet is typically loosened by turning the draw bar head ("knob") with a wrench. If the collet does not readily loosen, manual force must be applied to the wrench, often by hammering at the extended end of the wrench. Such force can result in damage to the machine. Moreover, the use of the wrench in rotating the draw bar is time-consuming and inefficient. In some applications the collet and cutting tool assembly must be changed from ten to twenty times each hour. Even if the change takes only fifteen seconds, the time lost can amount to three to six minutes each hour. It is thus desirable to provide a more rapid and efficient method for both loosening and tightening the draw bar onto a collet.
While attempts have been made in the past to provide a more efficient means for releasing and changing collets, each suffers from disadvantages. U.S. Pat. No. 3,481,248 discloses an entirely automatic device for changing collets. A pneumatic motor having a socket disposed above the driven end of the draw bar is mounted in a fixed position relative to tne draw bar. A solenoid operated clutch engages the motor and the draw bar when it is desired to change collets. Although functional, the device contains a large number of moving parts which increase its cost and are subject to failure.
U.S. Pat. No. 3,680,437 discloses an entirely manual device for engaging and rotating the upper end of a machine tool draw bar. The operator must depress a handle with one hand, while rotating a socket with the other hand in order to rotate the draw bar. Thus, there will be little saving of time by use of the disclosed device.
U.S. Pat. No. 3,898,911 discloses an electrically-driven impact wrench which is mounted on the piston of a piston and cylinder assembly located above the driven end of the draw bar. The piston is raised and lowered pneumatically, and, in its lowered position, allows the impact wrench to engage the draw bar. Both the impact wrench and the support piston are typically under the control of an automatic programmer. The mechanism used for controlling both the piston and the impact wrench is not described.
It is thus desirable to provide a device to enable the user of a manually-controlled milling machine to rapidly and with minimal effort change the cutting tool and/or the collet on the machine. Moreover, it is desirable that the device be inexpensive, rugged and reliable.